1. Field of the Invention
The present invention relates to a threaded connection between two metal pipes which may or may not be provided with an internal coating and is more particularly intended to contain a corrosive fluid.
2. Discussion of the Background
Threaded connections for metal pipe are already known for use in oil or gas wells such as production tubing to raise oil or gas to the surface, or as casing pipe to hold the ground around a well.
Such threaded connections are generally of two types:
a) integral, namely the end of a first pipe comprises a male connecting element and the end of a second pipe to be connected comprises a female connecting element. PA1 b) coupled, namely the ends of the pipes to be connected each comprise a male connecting element, each end being screwed to a short coupling comprising a female connecting element at each end. PA1 between two pipes of a metal which is resistant to corrosion by the fluid contained in them; PA1 and between two pipes of a metal which is not resistant to corrosion but is provided with an internal coating which is inert to the corrosive fluid; PA1 and between a pipe of a metal which is resistant to corrosion and a pipe provided with an internal coating. PA1 an annular transverse bearing surface formed over a fraction of the pipe thickness; PA1 the external peripheral surface of a nose with a reduced external diameter; PA1 an annular radial surface which joins the internal peripheral surface of the first pipe. PA1 an annular transverse bearing surface forming a shoulder which is complementary to the bearing surface of the male element, PA1 the internal peripheral surface of a zone termed a reinforcing zone; PA1 an internal peripheral housing surface for a sealing ring; PA1 an annular radial surface which joins the internal peripheral surface of the second pipe. PA1 an annular transverse bearing surface formed over a fraction of the pipe thickness; PA1 the external peripheral surface of a nose with a reduced external diameter; PA1 an annular radial surface which joins the internal peripheral surface of the pipe under consideration. PA1 an annular transverse bearing surface forming a shoulder which is complementary to the bearing surface of the male element; PA1 the internal peripheral surface of a zone termed a reinforcing zone; PA1 a housing surface for a sealing ring which surface is common with the housing surface of the other female element of the coupling.
French patent FR-A-1 489 013 describes an integral threaded connection with trapezoidal threads, the end of the male element having a concave conical shape which, at the end of the connection makeup, bears against a convex conical shoulder located at the base of the thread of the female element; further, the outer terminal edge of the male element is slightly chamfered and comes into mating contact against a corresponding concave truncated conical portion on the female element thus ensuring an excellent gas seal even after several successive uses of the connecting elements.
That patent also describes an option in which a connection for pipes is coated internally with a layer of enamel or epoxy resin when the fluid circulating in the pipes is aggressive or corrosive as regards the metal of the pipes.
Internally coated pipes may also be required when the fluid circulating in the pipes comprises water with chlorides and/or dissolved oxygen or CO.sub.2 and is thus highly corrosive towards the metal used.
In order to prevent any penetration of aggressive fluid into the shoulder, FR-A-1 489 013 proposes a housing on the inside of the male end for a complementary sealing ring which bears both on the coated zones of the male end and on the shoulder which is continuous with the internal coating of the pipes.
European patent application EP-A-0 759 497 describes a threaded connection which is intended for a water injection well to improve recovery of hydrocarbons from an oil field; the water injected into such a well is usually an oxygenated brine which is extremely corrosive.
EP-A-0 759 497 describes a threaded and coupled (T & C) connection, the coupling comprising internally, to either side of a protruding central portion, an annular shoulder followed by a female thread, the pipes comprising externally, moving towards their ends, a thread followed by an annular transverse bearing surface which is complementary to the shoulder of the coupling and finally a smooth zone terminated by an annular radial surface.
A sealing ring of soft synthetic material is interposed between the ends of the pipes and is compressed by the ends of the pipe, the position of which at the end of the connection makeup being determined by the thrust of the bearing surfaces of the pipes on the corresponding surfaces on the coupling. The internal surface and the annular end surfaces of the pipe are coated with a synthetic resin such that penetration of corrosive liquid into the metal parts of the connection is prevented by the compressed ring.
EP-A-0 759 497 also describes solutions for integrally threaded connections derived directly from the solution described for the T & C connection, the bearing surface in this case being moved to the free end of the female element.
Such a connection with a sealing ring of soft synthetic material cannot be completely tight, in particular gas tight, as the contact pressure exerted by the soft material ring is much lower than that resulting from elastic deformation of a metal on a metal as described in FR-A-1 489 013.